Caption: A view of the PETRA particle collider at DESY, the German particle physics laboratory in Hamburg. Built in 1976- 1978 in an underground tunnel 2. 3 km in circumference, PETRA collided beams of electrons & positrons at energies of up to 23. 5 GeV per beam. It consists of 224 5. 8 metre long bending magnets, which guide the electrons & positrons in opposite directions around the ring, & some 500 focusing & other magnets. In 1978, PETRA was converted into PETRA II (seen here), which will act as a booster accelerator for injecting particles into DESY's new HERA accelerator, which will collide electrons & protons.